The present invention generally relates to servo systems and, more particularly, to compensating phase errors in a position control signal in a servo position estimation system.
Computer disc drives store information on discs or platters. Typically, the information is stored on each disc in tracks. The data tracks are usually divided into sectors. Information is written to and read from a storage surface(s) of a disc by a read/write head. The read/write head may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The read/write head is mounted on an actuator arm capable of moving the read/write head radially over the disc. Accordingly, the movement of the actuator arm allows the read/write head to access different data tracks.
The disc is rotated by a spindle motor at a high speed, allowing the read/write head to access different sectors within each track on the disc. The actuator arm is coupled to a motor or coarse actuator, such as a voice coil motor (VCM), to move the actuator arm such that the read/write head moves radially over the disc. Operation of the coarse actuator is controlled by a servo control system. The surface of the disc contains a plurality of data tracks that are divided into a plurality of data sectors. In addition to the data sectors, servo control information is embedded on the disc. The embedded servo information, which is typically included in servo sectors that are adjacent to the data sectors, is used to determine the location of the read/write head on the disc. A servo sector generally includes a track identification (ID) field and a group of servo bursts that the servo control system samples to align the read/write head with or relative to a particular data track.
The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the read/write head is moved from an initial position to a target track position using the track ID field as a control input.
In general, the seek function is initiated when a host computer associated with the disc drive issues a command to read data from or write data to a target track on the disc. Once the read/write head has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the servo control system is activated to center and maintain the read/write head on the target track until the desired data transfers are completed.
Once the read/write head is over the desired track, the servo control system uses the servo bursts in a “track following” mode to keep the read/write head over that track. That is, the servo control system attempts to maintain the position of the read/write head over a centerline of the track. Typically, the read/write head must be moved slightly during track following to maintain a desired position over the track, because the track may not be perfectly circular on the disc and/or there may be some eccentricity in the rotation of the disc about the disc spindle.
The servo bursts typically include groups of discrete servo bursts that are formed in succession on a track and are disposed at predetermined radial positions on the disc relative to a track centerline. Some of the bursts may be regarded as in-phase bursts, while other bursts are radially offset from and overlap the in-phase bursts, so that they are located in quadrature. Signals generated in response to the servo bursts can be used to determine displacement of the read/write head from the centerline of the data track using various servo demodulation schemes. Various schemes of servo position encoding have been used in the industry including split-burst amplitude patterns, null servo patterns, phase patterns, dual-frequency patterns etc.